The invention pertains to modems. More particularly, the invention pertains to modems designed to operate in accordance with the CCITT V.90 and V.90PLUS standardized protocols.
Modems are transceiver devices that allow digital data to be transmitted between pieces of digital equipment, e.g., computers, via the telephone lines. The transmitting modem receives serial digital data from a computer (typically passed from the computer to the modem through a UART (Universal Asynchronous Receiver Transmitter) in order to convert it from parallel to serial format. The modem converts the data to a signal form that can be transmitted effectively via the public telephone system. The receiving modem receives that data and converts it back to serial digital format and passes it to the receiving computer (typically through another UART, which converts the data back to parallel).
Over the past few decades, several protocol standards for modems have been developed. One of the more recent standards has been promulgated by the ITU (International Telecommunications Union) formerly known as the CCITT and is known as ITU-T recommendation V.90, incorporated herein by reference. Earlier generation standards developed by the ITU/CCITT include V.22, V.22bis, V.32, V.34, V.42 and V.42bis.
In the relevant industries, communication in the transmit direction from a network node, such as a telephone or a modem, in the direction of the telephone company central office is termed the upstream direction. Receive direction communications from the network towards a node is termed the downstream direction. In accordance with the V.90 protocol, the data format is different in the downstream direction than it is in the upstream direction. In the V.90 standard, modem transmission in the upstream direction is an analog signal in accordance with the older V.34 standard. However, downstream communication is a PCM (pulse code modulated) signal.
There also is a proposed V.90PLUS recommendation, also incorporated herein by reference, which presently is not in commercial use. In the V.90PLUS standard, PCM is used in both the upstream and downstream directions.
FIG. 1 is a block diagram generally illustrating modem to modem communications through a public telephone network. The system will be described in connection with a public telephone network customer exchanging data with his Internet service provider (ISP) through the public telephone network. For purposes of fully illustrating the various factors contributing to noise in this type of communication, let us assume the customer and his ISP are coupled to different central offices of the public telephone network.
The customer at computer 12 inputs and sends data to the ISP at 28. The computer 12 includes a built-in UART and, therefore, sends out a serial digital signal to the modem 14. The modem converts the serial digital signal to comply with the V.90 standard (which, in the upstream direction, is the analog V.34 standard) and puts it out on the public telephone network 20.
Under the V.34 standard, data rates as great as 31.2 kilobits per second (Kbps) can be achieved.
Within the telephone network, telephony communications between central offices are digital, rather than analog. Accordingly, the analog signal is encoded by a codec 22 into a 64 Kbps signal. In particular, the received analog signal is sampled at a rate of 8 KHz and digitized at an 8 bit resolution to produce a 64 kbps digital PCM signal. The 64 kbps standard is known in the United States as the xcexc-law standard and in Europe as the A-law standard. The information is digitally transmitted between central office 24 and central office 26.
For voice and data communications between two normal customers of the public telephone network, the digital signals received at central office 26 from other central offices on the network, e.g., central office 24, would be passed through another codec (not shown) to be decoded back to analog form. The decoded analog signals would then be forwarded to the receiving customer. However, a high volume customer of the public telephone network, such as an ISP 28, would likely have a high bandwidth digital connection to the central office 26, such as a T-1 line 30. Accordingly, ISP 28 would not use a codec in central office 26, but instead would receive the data in digital form over a digital link, such as a T1 line 30.
In the opposite direction, ISP 28 outputs digital data to central office 26 via T1 line 30. This data is transmitted in digital form to central office 24. Codec 22 in central office 24 decodes the digital data to a PCM analog version of the digital signal in accordance with the V.90 protocol and transmits it to the customer. The customer""s modem 14 receives the data and converts it to a serial digital data format detectable by computer 12. Finally, the UART in computer 12 converts the data from serial to parallel. In the downstream direction, data can be received at rates as great as 56 Kbps.
As can be seen, under the V.90 standard, upstream communications are at a different data rate, i.e., 31.2 Kbps, than downstream communications, i.e., 56 kbps. Further, the communications in the upstream direction, is in an analog format, i.e., V.34, and, in the downstream direction, are in PCM format.
FIG. 2 is a more detailed block diagram of the interface between a customer""s modem 14 and the local central office 24. As shown, the modem 14 accepts transmit data from the computer""s UART 201 on a transmit data path 202 and sends data to the computer""s UART 201 on a receive data path 204. In the transmit (upstream) direction, V.90 transmitter 203 in modem module 206 of modem 14 converts data between the serial digital format generated by the UART 201 to the analog V.34 format. Codec 209 converts the data from digital to analog for transmission over the telephone lines. In the receive direction (i.e., downstream), V.90 receiver 205 in modem module 206 converts data from the V.90 PCM format to the serial digital format used by UART 201. Codec 209 converts data from analog to digital in the receive direction.
The customer""s equipment (to the left of hybrid circuitry 208 in FIG. 2) is a four wire system. That is, there are two wires for the transmit direction (i.e., each of lines 202 and 204 comprises two wires) and two wires for the receive direction (i.e., each lines 204 and 207 comprises two wires). The public telephone network, however, is a two wire system in which the transmit data and the receive data are transmitted over the same wire pair. Accordingly, a hybrid circuit 208 interfaces between the codec 209 and the public telephone network 210. In the transmit direction, it takes the transmit data from the codec and places it on the two wires 211 (tip and ring) of the telephone network. In the receive direction, it selects and isolates the receive data from wires 211 and forwards it to the modem module 206 on the receive wire pair 207. There is almost always an impedance mismatch between the customer""s telephone equipment and the public telephone network. This impedance mismatch has the unfortunate effect of causing an echo at the hybrid circuit. The echo occurs in both directions. For instance, data transmitted from the computer 12 through the modem module 206 to the hybrid 208 is reflected back on the receive wire pair 207 to the modem module 206 and computer 12. Likewise, data received from the public telephone network over the tip and ring wire pair 211 also is reflected back onto the public telephone network.
At the central office, there is another hybrid circuit 224 and codec circuit 226 serving essentially the same functions. Hybrid circuit 224 also creates echos in both directions. The echo from hybrid circuit 224 passes back through hybrid circuit 208 and reach the receive data path 204, 207. Such echoes are not particularly bothersome for voice communications, which can bear a significant amount of noise and still provide signal quality acceptable to the human ear. However, echo signals of large enough amplitude can corrupt digital data that is being received simultaneously with the echo on the receive data path 204, 207.
FIG. 3 is a block diagram illustrating a customer-to-customer link through a public telephone network between an individual using a PC with a V.90 modem and his ISP having an all digital connection to the network. FIG. 3 illustrates echo effects.
In this example, the two customers are geographically distant from each other so that they are coupled to different central offices. Accordingly, transmissions in the upstream direction pass from the first customer""s transmission circuitry 302 over transmit data path 303 through his hybrid circuit 304 onto two wire portion 316 of the public telephone network and through the hybrid circuit 306 in central office 308 to re-separate the transmit and receive direction data for the four wire digital network portion 310. The data is converted to digital and then transmitted over the digital, inter-central-office network portion 310 to central office 312 where it is forwarded, still in digital form, to modem 314 of ISP 316.
Thus, the first customer""s telephone equipment has a hybrid circuit 304 for converting from four wire to two wire. His local central office also has a hybrid circuit 306 for converting from two wire back to four wire for the digital network 310. In the upstream direction, the customer experiences a near echo 333a from hybrid circuit 304 and a near echo 333b from hybrid circuitry 306. Because the hybrid circuit 304 in the customer""s own equipment as well as the hybrid circuit 306 in his local central office are physically close to him, the near echo is almost simultaneous with the actual transmission of the data. Accordingly, in most circumstances it can be ignored without significant adverse effect in voice communication. However, in data communication, a near echo canceller is required in client modem 301 to remove the near echo in order to achieve better performance.
The ISP""s modem 314 experiences a far echo 333c from hybrid circuit 306 in far central office 308 and a far echo 333d from the customer""s hybrid circuit 304. Receipt of the far echos at a modem such as modem 314 may be, and commonly are, sufficiently delayed from the original transmission of the data that created the echo to corrupt data on the receive data path of modem 314.
In order to minimize the effect of far echo, therefore, a digital loss of approximately 6 decibels (dB) is incorporated into hybrid circuits so as to reduce echo amplitude. However, even with the incorporation of digital loss, far echo sometimes can still create sufficient noise to corrupt data.
Thus, in order to further compensate for echo, digital communications equipment (e.g., modems) commonly include an echo canceller circuit. FIG. 4 is a block diagram of an echo canceller circuit of the prior art. The transmit signal from transmitter 400 on transmit path 401 is fed out to the digital network 402. The transmit signal also is fed into an echo cancellation circuit 403. The echo canceller circuit includes a bulk delay line buffer 404 and a Finite Impulse Response (FIR) circuit 406. FIR circuit 406 receives the transmit signal from transmit wire pair 401 through bulk delay line buffer 404 and generates an echo cancellation signal that can be used to cancel the far echo signal portion that returns from the network. The FIR circuit determines, at the beginning of each call, the channel response for the call (e.g., attenuation of echoes, etc.), emulates it and applies it to the data transmitted from transmitter 400 so that the echo cancellation signal emulates the echo signal. The bulk delay line buffer 404 is the circuit that determines and causes the necessary delay in order to cause the output from the FIR circuit 406 to be simultaneous with the receipt of the far echo.
As is well known in the art, significant handshaking takes place between the central office interface circuit and the customer""s modem. From that handshaking, the round trip delay of the far echo as well as the channel response for any given telephone call can be readily determined. Accordingly, a processor 412 in the modem determines the round trip delay and the necessary coefficients for the FIR circuit 406 from the handshaking data and sends the data to the bulk delay line buffer 404 and the FIR circuit, respectively. The delay circuit 404 will then delay passing the transmit data from transmit path 401 to the FIR circuit 406 for the appropriate duration, namely, the round trip delay, and the FIR will attenuate and otherwise condition the transmit signal to emulate the echo signal. Subtractor 410 subtracts the output of FIR circuit 406 from the receive data path 408 in order to cancel the far echo component that appears on receive data path 408. It should be noted that the far central office and the receiving customer""s equipment are typically geographically close to each other such that the difference in delay between the two can be ignored and the far echo treated as a single far echo signal.
Another noise factor inherent in telephony communications is xe2x80x9crobbed bitxe2x80x9d noise. In particular, in the digital network portion between telephone company central offices, the least significant bit (LSB) of every sixth data sample is utilized for synchronization. In the United States, for instance, there is one type of robbed bit loss, termed type A. In type A robbed bit systems, the LSB of every sixth data sample (each data sample comprises 8 bits) is forced to digital one regardless of the actual data content. There also are other types of robbed bit protocols. Further, if a connection is routed through a plurality of central offices between the two termination points of the connection, a robbed bit may be inserted for each central office through which a particular call is routed such that there may be several robbed bits every six samples. As will become clear from the discussion below, the present invention is applicable regardless of the particular robbed bit protocol utilized or the number of robbed bits inserted.
In voice communications, for which, of course, the telephone network was originally constructed, the loss of that bit is imperceptible to the listener and, therefore, unimportant. The echo effect of the robbed bit also is acceptable in connection with analog data transmissions such as in accordance with the V.34 modem standard. However, in PCM data communications over the telephone network, the robbed bit must be taken into consideration. Particularly, data cannot be sent in that bit position since it will be corrupted in the digital portion of the network.
Further, the far echo that comes back through the digital network includes the robbed bit. Accordingly, the echo cancellation signal generated by echo cancellation circuit 403 will not exactly match the echo signal portion. Specifically, the signal echoed back to the transmitting equipment contains the robbed bit, whereas the signal that was transmitted on transmit path 401, and, therefore, was used to create the echo cancellation signal did not contain the robbed bit.
However, the robbed bit is generated in the inter-central-office digital portion of the telephone network. Accordingly, the PCM output signal from the central office codec does not include the robbed bit, which is added later. Accordingly, the signal that is sent from the central PCM modem to the echo canceller circuit does not include the robbed bit information. Accordingly, the echo canceller cannot cancel the robbed bit which is received in the echo.
Accordingly, it is the object of the present invention to provide an improved far echo cancellation method and apparatus.
In accordance with the present invention, a PCM modem is provided with a far echo canceller circuit which includes a robbed bit generator to compensate for the robbed bit which will appear in the far echo signal received over a telephone network.
In particular, during the handshaking which occurs at the initiation of a telephone call, the transmitting PCM modem determines from the modem at the receiving end the position of the robbed bit added by the telephone network. Based on this information, the position of every robbed bit during the telephone call is known, since it occurs at regular intervals. The robbed bit position information is provided to a robbed bit generator circuit in the echo canceller of the PCM modem which then incorporates the robbed bit into the echo cancellation signal to compensate for the far echo signal, including the robbed bit.